Ammonia levels in water are a result of metabolism by stock and the decomposition of organic material in the water by bacteria. Unlike many terrestrial animals that produce nitrogenous waste in the form of urea, aquatic stock excrete their nitrogenous waste in the form of ammonia. Excessive feeding of stock or a stocking density that is too large for the culture system can result in elevated levels of ammonia.

In water, ammonia exists in two forms:

Ammonia (NH3), which is toxic to fish, and
Ammonium (NH4+), which is relatively non-toxic.

When measured together these are termed the
'Total Ammonium Nitrogen' (TAN) level. The toxic levels for ammonia are usually between 0.6 and 2.0 ppm for short term exposure although levels of as low as 0.01 ppm can affect the growth rates of fish.

The percentage of ammonia and ammonium varies in the water as a result of the pH and temperature. The relationship between ammonia and ammonium can be seen in the following ammonia-ammonium equilibrium equation:
While the amount of ammonia and ammonium remains the same at any one time, the percentage of ammonia and ammonium can change as a result of the pH and temperature.

Below is a graph showing the percent of Un-ionised (toxic) Ammonia (NH3) present under varying temperatures and pH.


We measure total ammonia-nitrogen (TAN), that is ammonia plus ammonium. There is no test kit available that measures un-ionized ammonia, however it is possible to determine the actual ammonia level if we know (a) TAN, (b) the water temperature and (c) the water pH.

If your system is running at a pH of 6.8 and the water temperature is 15oC then the Ammonia NH3 level is 0.17%, however if you then raise the pH to 8.4 then the percentage of un-ionised Ammonia ( NH3) level will rise to 6.44%. which may be still be OK depending on the total ammonia (TAN) , but if the temperature then rises to 25oC then the NH3 level will be 12.5% which could well be outside the acceptable levels and the system will be toxic to your fish.

Table 1 % un-ionised ammonia

The table 2 below shows the maximum acceptable level of TAN at a given pH and temperature. For example at pH 7.5 and a water temperature of 20oC, a TAN of 2 mg / litre would be fairly safe as only 1.2% would exist as un-ionised ammonia (0.024 mg/litre). If the actual TAN measurement is greater than 2mg/L, then the level is unsafe.
Table 2 Maximum TAN

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